KM offers a limited Action for doing screen shots in the OCR Image action, but for a much fuller feature set you can use the native macOS tools.
Here's a good article by Macworld to show you how.
Here’s a fairly full refresh including a few secrets I only learned days ago from Twitter.
In macOS 10.14 Mojave, Apple got rid of the long-running Grab app, a screen-captured utility with several features beyond what you could manage with keystrokes. Instead, it embedded most of those features into a new shortcut.
See the article for full details.